1. Field of the Invention
The present invention relates to a digital displacement measuring instrument for measuring a dimension of an object to be measured based on an axial displacement of a spindle.
2. Description of Related Art
A digital micrometer, one of widely-used digital displacement measuring instrument, includes a body, a spindle screwed into the body in a manner capable of axial advancement and retraction, an encoder for detecting a displacement of the spindle and a display for digitally displaying the displacement of the spindle calculated based on a detected value of the encoder.
The encoder has an opposing arrangement of a stator fixed onto the body and a rotor capable of synchronized rotation with the spindle, where the rotation (angle) of the rotor relative to the stator is detected to measure the displacement of the spindle relative to the body.
An arrangement of a rotor section 41 of a conventional digital micrometer is shown in FIGS. 5 and 6. The rotor 43 is supported by a rotor support member 440 having an engaging key 47. An engaging groove 23 is provided on an outer circumference of a spindle 2 in the axial direction thereof, the engaging key 47 being engaged with the engaging groove 23. Accordingly, the rotor 43 is held at a fixed position to keep a constant gap against the stator 42 irrespective of the axial movement of the spindle 2.
The spindle 2 is advanced and retracted in the axial direction relative to the digital micrometer body while being rotated in circumferential direction. Then, the rotor support member 440 is also rotated in synchronization with the spindle 2, so that the rotary angle of the rotor 43 relative to the stator 42 is detected, which is converted into the displacement of the spindle 2 to be digitally displayed.
As described above, a digital micrometer employs a rotation-transmitting mechanism that transmits the rotation of the spindle while keeping a constant gap between the stator and the rotor, which includes the combination of an engaging groove formed on the outer circumference of the spindle in the axial direction, an engaging key adapted to be engaged into the engaging groove and a rotor support member having the engaging key.
However, a rotary transmission error may be caused in accordance with depth position of the engaging key in such rotation-transmitting mechanism.
For instance, when there is a gap between the tip end of the engaging key and the engaging groove, a loose area in which the rotation of the spindle is not transmitted to the engaging key is generated in the gap, which results in the rotation transmission error. On the other hand, smooth advancement and retraction of the spindle is impaired when the engaging key is firmly pressed onto the engaging groove.
In order to avoid the above disadvantages, the depth position of the engaging key has to be accurately adjusted at a position that allows contact between the tip end of the engaging key and the engaging groove while avoiding excessive pressure on the engaging groove by the engaging key. However, such adjustment requires very accurate work and difficulties.
In view of the above, a new digital displacement measuring instrument has been developed, where an engaging key is pressed toward the engaging groove and the engaging key and the engaging groove are engaged without gap with the use of a plate spring and the like (document: U.S. Pat. No. 7,013,576).
However, in the above document, workability for assembling the rotor section and the engaging key onto the spindle is not so excellent.
In order to install the rotor section and the engaging key onto the spindle, following two methods can be considered.
The first method is to assemble a rotor section onto a spindle, and, subsequently, an engaging key and a plate spring are installed onto a rotor support member. Since the rotor section is liable to rotate in the axial direction of the spindle in the above method, it is necessary to conduct the assembling work while pressing the rotor section to prevent rotation, which considerably deteriorates workability.
In the second method, after an engaging key and a plate spring is installed on a rotor support member, a rotor section is assembled onto the spindle. In this method, however, the engaging key is likely to be dropped off before installation.